1. Field of the Invention
The present invention relates to a device for detecting the absolute angle of multiple rotations such as, for example, the absolute steering angle of steering, to a bearing assembly equipped with such an absolute angle detecting device and to a method of detecting the angle thereof.
2. Description of the Prior Art
The above-mentioned type of multiple rotation absolute angle detecting device has been suggested in the art, in which using a reduction gear mechanism made up of a combination of a worm and a worm wheel or a reduction gear mechanism utilizing a set of spur gears, output signals outputted respectively from an encoder for detecting an absolute angle within one complete rotation and an encoder for detecting multiple rotation are synthesized by a signal processing circuit to thereby detect the absolute angle of multiple rotations. (See, for example, the Japanese Laid-open Patent Publications No. 2001-289671 published Oct. 19, 2001 and No. 2004-45083 published Feb. 12, 2004.)
As a device for use in detecting the steering angle of a steering wheel, a device for detecting the rotation angle of a steering shaft has also been suggested, in which while a large diameter gear is employed in coaxial relation with the steering shaft, in combination with a reduced diameter gear drivingly engaged with the large diameter gear, the rotation angle of the steering shaft can be determined based on an output signal from a reference position sensor for detecting magnets, mounted on an outer peripheral portion of the large diameter gear in 90° spaced relation to each other, and an output signal from a magnetic sensor for detecting lines of magnetic force emanating from a rotation angle detecting magnet mounted on the reduced diameter gear. (See, for example, the Japanese Laid-open Patent Publication No. 2003-344009 published Dec. 3, 2003.)
Furthermore, a multiple rotation absolute angle detecting device of a structure, which is so designed as to detect the absolute angle of multiple rotations of a to-be-measured member, which is coupled direct with an input shaft of a planetary gear mechanism, through detection of rotation of an output shaft of the planetary gear mechanism. (See, for example, the Japanese Laid-open Patent Publication No. 2002-340545 published Nov. 27, 2002.)
Yet, as a mechanism for detecting the rotation angle as an absolute angle, the device has been suggested in the art, which includes a to-be-detected member fitted to a rotatable raceway member of a bearing assembly and having a magnetic characteristic varying, with one complete rotation taken as one cycle, and a magnetic sensor fitted to a stationary raceway member of the bearing assembly in face-to-face relation with the to-be-detected member and which is so designed as to detect the absolute rotation angle without any initialization being carried out at the time the device is electrically powered on. (See, for example, the Japanese Laid-open Patent Publication No. 2004-4028 published Jan. 8, 2004.)
As a mechanism for detecting the absolute angle of multiple rotations, the device has been suggested in the art, which includes a to-be-detected member fitted to a retainer of a bearing assembly and having a magnetic characteristic cyclically varying, with one complete rotation taken as one cycle, and a magnetic sensor fitted to a stationary raceway member of the bearing assembly in face-to-face relation with the to-be-detected member and which is so designed as to detect the absolute rotation angle of the rotatable raceway member over multiple rotations, taking advantage of a reduced rotational speed of the retainer relative to the rotatable raceway member. (See, for example, the Japanese Laid-open Patent Publication No. 2004-308724 published Nov. 4, 2004.)
However, the device, in which the reduction gear mechanism including a combination of the worm and the worm wheel such as disclosed in the Japanese Laid-open Patent Publication No. 2001-289671 referred to above, has a problem in what although a relatively large reduction gear ratio can be enjoyed, not only is the structure complicated because of the shafts lying perpendicular to each other, but a relatively large space is required.
Also, the device, which utilizes the reduction gear mechanism including the spur gears such as disclosed in the Japanese Laid-open Patent Publications No. 2004-45083 and No. 2003-344009 referred to above, requires a relatively large space in a radial direction of the mechanism for accommodating the sensor. Moreover, since it makes use of a plurality of shafts, a relatively large space is required for installation of the bearing assembly, resulting in complicating of the structure.
According to the structure, which utilizes the planetary gear mechanism such as disclosed in the Japanese Laid-open Patent Publication No. 2002-340545 referred to above, a problem has been found as to how the gears (particularly, planetary gears) are to be supported as well as the increased number of the gears employed. Additionally, an attempt to increase the reduction gear ratio tends to result in increase of the dimensions in a direction radially of the mechanism.
Further, the multiple rotation absolute angle detecting device utilizing the various reduction gear devices discussed above is capable of detecting only the number of rotations corresponding to the reduction gear ratio (for example, only five complete rotations where the reduction gear ratio is 1/5).
Furthermore, the device disclosed in the Japanese Laid-open Patent Publication No. 2004-4028 referred to above is capable of detecting only the absolute angle within one complete rotation.
Also, the device disclosed in the Japanese Laid-open Patent Publication No. 2004-308724 referred to above, in which the to-be-detected member is fitted to the retainer, is incapable of accurately detecting the absolute angle since during the rotation or the halt and start of rotation, rolling members often tend to undergo slippage.